The invention relates to a superconducting device having a rotor which is mounted such that it can rotate about a rotation axis and has at least one superconducting winding, whose conductors are arranged in a thermally conductive winding mount, and having a cooling unit which has at least one cooling head that is thermally coupled to the winding. A corresponding device is disclosed in U.S. Pat. No. 5,482,919 A.
In addition to metallic superconductor materials such as NbTi or Nb3Sn which have been known for a very long time and have a very low critical temperatures Tc and are therefore also referred to as low-Tc superconductor materials or HTS materials, metal-oxidic superconductor materials have been known since 1987, with critical temperatures above 77 K. The latter materials are also referred to as high-Tc superconductor materials or HTS materials and, in principle, allow a cooling technique using liquid nitrogen (LN2).
Attempts have also been made to produce superconducting windings by conductors using such HTS materials. However, it has been found that already known conductors have only a comparatively low current-carrying capacity in magnetic fields with inductions in the Tesla range. This often makes it necessary for the conductors of such windings nevertheless to have to be kept at a temperature level below 77 K, for example between 10 and 50 K, despite the intrinsically high critical temperatures of the materials used, in order to make it possible to carry significant currents in this way in field strengths of several Tesla. A temperature level such as this is admittedly on the one hand considerably greater than 4.2 K, the boiling temperature of the liquid helium (LHe) with which known metallicauperconductor materials such as Nb3Sn are cooled. On the other hand, however, cooling with LN2 is uneconomic owing to the high conductor losses. Other liquefied gases such as hydrogen with a boiling temperature of 20.4 K or Neon with a boiling temperature of 27.1 K cannot be used, owing to their danger or owing to their lack of availability.
Cooling units in the form of cryogenic coolers with closed helium compressed-gas circuits are therefore preferably used for cooling windings with HTS conductors in the stated temperature range. Cryogenic coolers such as these are, in particular, of the Gifford-McMahon or Stirling type, or are in the form of so-called pulsed-tube coolers. Cooling units such as these also have the advantage that the cooling performance is available just by pushing a button, avoiding the need for the user to handle cryogenic liquids. When using cooling units such as these, a superconducting device such as a magnet coil or a transformer winding is cooled only indirectly by thermal conduction to a cooling head of a refrigerator (see, for example, “Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16)”, Kitakyushu, J P, 20.-24.05.1996, Publisher Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also envisaged for the superconducting rotor of an electrical machine as disclosed in the initially cited US-A document. The rotor contains a rotating winding composed of HTS conductors, which can be cooled to a desired operating temperature of between 30 and 40 K by a cooling unit in the form of a Stirling, Gifford-McMahon or pulsed tube cooler. For this purpose, one specific embodiment of the cooling unit contains a cooling head which also rotates but is not described in any more detail in the documents, and whose cold side is thermally coupled indirectly to the winding via thermally conductive elements. The cooling unit of the known machine also contains a compressor unit which is located outside its rotor and supplies the cooling head with the necessary operating gas via a rotating coupling, which is not described in any more detail, of a corresponding transfer unit. The coupling also supplies the necessary electrical power to a valve drive (which is integrated in the cooling head) for the cooling unit, via two sliprings. This concept makes it necessary for at least two gas connections to be routed coaxially in the transfer unit, and means that at least two electrical sliprings must be provided. Furthermore, the accessibility to those parts of the cooling unit which also rotate, and in particular to the parts of the valve drive in the rotor of the machine, is impeded since the rotor housing must be opened for the necessary maintenance operations. In addition, the operation of a known valve drive is not ensured at high rotation speeds, such as those which occur in synchronous motors or generators.
Against the background of the related art, one possible object for the present invention is to refine the device having the features mentioned initially such that it allows the cooling unit to be operated reliably, safely and economically both when at rest and when the rotor is rotated in a temperature range below 77 K, with comparatively less hardware complexity.
The superconducting device accordingly comprises a rotor which is mounted such that it can rotate about a rotation axis and has at least one superconducting winding, whose conductors are arranged in a thermally conductive winding mount, as well as a cooling unit which has at least one cooling head that is thermally coupled to the winding. In this case, the superconducting device is intended to have the following features, namely
that the winding mount is equipped with a central, cylindrical cavity which extends in the axial direction,
in that the cooling head is located in a fixed position outside the rotor and is rigidly and thermally conductively connected to a heat transmission cylinder, which projects into the cavity of the winding body while maintaining a hollow cylindrical annular gap, and
in that the annular gap is filled, at least in the area of the winding mount, with a contact gas for heat transmission between the winding mount and the heat transmission cylinder, and is sealed in a gastight manner.
In consequence, in the refinement of the superconducting device, the entire cooling unit is arranged with its possibly moving parts in a fixed position outside the rotor, and is thus easily accessible at all times. The cooling performance and the heat transfer are provided by a fixed cooling finger in the form of the heat transmission cylinder, which is thermally highly conductively connected to the cooling head, by the gas flow of the contact gas to the rotating winding mount. In this case, one advantageous feature is that no forced circulation of the contact gas is used; instead, the rotation of the rotor together with the centrifugal forces in the contact gas ensures convection of the gas. Furthermore, even when the rotor is stationary, the convection that occurs in the contact gas makes it possible to cool down from room temperature to low temperature or to maintain the low temperature conditions in the rotor. This is a consequence of the chosen geometry of the structure of the heat transmission cylinder in the cylindrical cavity while maintaining the annular gap. The heat transfer and the provision of the cooling performance with this structure are particularly simple and economic and, furthermore, only a comparatively simple seal is required for the annular gap.
The annular gap can thus be sealed particularly easily if the cavity is closed on one side by the winding mount and a sealing device with parts that also rotate is provided on the side facing the cooling head. In this case, at least one seal from the group of ferrofluid seal, labyrinth seal, gap seal may preferably be used as the sealing device.
Virtually all types of cooling unit may be provided which have a cooling head which can be reduced to a predetermined temperature level. Cryogenic coolers are preferably provided, in particular with a closed helium compressed-gas circuit, since these have a simple design and are particularly suitable for an indirect cooling technique such as that used for the superconducting device. Appropriate coolers, which are also referred to as regenerative cryogenic coolers, have a regenerator or regenerative operating cycle corresponding to the normal classification for cryogenic coolers (see, for example, the cited Proceedings volume, pages 33 to 44).
It is particularly advantageous for the cooling head to have a plurality of stages. Parts of an electrical power supply or a thermal radiation shield can be reduced to a comparatively high intermediate temperature by its first stage. An appropriately designed cooling head thus in each case allows even stationary parts of a semiconductor device to be kept at a temperature level that is suitable for effective cooling, in a simple manner.
It may also be regarded as being advantageous for the winding to be cooled and hence its superconductor material to be kept at a temperature below 77 K by the cooling head, and preferably at between 20 and 50 K when using HTS material. This is because known HTS materials have a critical current density which is sufficient for normal applications in this temperature range, which can be maintained with a relatively restricted amount of cooling. The necessary cooling power can be applied without any problems for the superconducting device. By way of example, it is in the range of a few tens of watts at 20 K to 30 K for a synchronous machine in the order of magnitude of a mechanical power of about 1 to 20 MW.